Phosphogypsum is the by-product from the manufacture of wet-process phosphoric acid by the so-called dihydrate process. Phosphogypsum is composed primarily of (i.e. contains at least 75%) calcium sulfate dihydrate (CaSO.sub.4. 2H.sub.2 O), which is commonly known as gypsum. Phosphogypsum is usually contaminated with small concentrations of impurities, such as phosphate, fluorine, and silica. This industrial by-product is generated in huge quantities and its disposal is a major problem in most areas where it is produced. Current worldwide production of phosphoric acid yields over 100 million tons of phosphogypsum per year. Because of environmental constraints, it is becoming increasingly difficult to discharge phosphogypsum into the seas or rivers, as has been done at many locations in the past. As a result of these disposal problems, most phosphogypsum currently produced in the United States is stockpiled. In Florida, a region of significant wet-process phosphoric acid production, the countryside near the wet-process phosphoric acid plants is liberally sprinkled with large mounds of the by-product phosphogypsum.
A small portion of the phosphogypsum produced is used as a substitute for natural gypsum The amount of phosphogypsum used is severely limited because of large amounts of the inexpensive natural gypsum readily available in most areas. Additionally, the phosphogypsum contains impurities which impair the qualities of the products produced from the phosphogypsum. Typically, the impurities must be removed by costly purification processes before the phosphogypsum can be used in some applications as a substitute for natural gypsum which is a material of higher purity.
Some attempts have been made to utilize phosphogypsum as base and fill materials in the construction of highways, runways, levees, etc. Some of the physical and chemical properties of phosphogypsum would appear to make it particularly useful for some of these applications. Its relatively low density should make phosphogypsum useful as a levee fill material in certain areas, e.g., Louisiana, which are known for their deep, highly water-saturated clay soils. For example, U.S. Pat. Nos. 4,299,516 and 4,306,910 disclose methods for using gypsum mixed with other ingredients for strengthening water saturated soft soils. The soil is first mixed with gypsum to make the soil reactive with the second component. The second component, which includes ordinary Portland cement and iron ore blast furnace slag, is mixed with the soil mixture and the resultant mixture is cured. The cured composition is used as a fill and base material
U.S. Pat. No. 3,854,968 discloses a cementitious mixture of gypsum, lime and fly ash suitable for stabilizing soils and for use as base materials.
U.S. Pat. No. 4,353,749 describes a process wherein phosphogypsum, an alkaline waste such as the by-product of digesting bauxite with caustic soda, Portland cement and/or fly ash, and water are blended and compacted to produce a soil cement product suitable for highway pavement.
U.S. Pat. No. 4,448,566 discloses a method of making a load-bearing surface by blending phosphogypsum and fly ash and spraying the mixture with water. The blended material is then compacted to a predetermined density. The mixing, spraying with water and compacting steps are repeated several times before the material is allowed to cure to produce a sub-base and base for pavement and the like.
French Patent No. 2,340,405 describes another paving composition containing phosphogypsum having a moisture content of between 8% and 23%.
U.S. Pat. No. 4,067,939 describes a method for making cast articles from a composition comprising a mixture of calcined gypsum and Portland cement with water to produce a fluid mixture. The calcined gypsum is dehydrated by means of heat and is defined as calcium sulfate hemihydrate having a chemical formula CaSO.sub.4.1/2H.sub.2 O. The method of this patent suffers from the distinct disadvantage of requiring that the gypsum must be calcined prior to its use, so as to convert the calcium sulfate dihydrate to a different chemical entity, namely calcium sulfate hemihydrate.
The processes described above have not ushered in an era of extensive phosphogypsum use, and the potentially valuable material continues to accumulate. A major problem with the use of the above disclosed phosphogypsum compositions in construction uses, such as for roadway bases, has been the excessive swelling or expansion of the phosphogypsum composition, resulting in heaving of the roadway. Also, the expansion often results in decompaction, the formation of cracks, and a loss of compressive strength. Calcination of phosphogygsum prior to its use in civil engineering application appears economically prohibitive.
A need obviously exists in the industry for a method of utilizing phosphogypsum in an economically and environmentally acceptable manner. A need also exists in the industry for a method of minimizing the expansion or swelling of phosphogypsum compositions which utilize cementitious binders in order that such compositions can be utilized as construction materials. It is therefore the object of this invention to provide a method of producing stabilized phosphogypsum compositions which contain cementitious binders and which exhibit acceptable volumetric stability characteristics and meet other typical construction specifications. It is a further object of this invention to provide a method of stabilizing phosphogypsum for use in the construction industry without the need for prior calcination of the phosphogypsum.